Where old Debian repositories are stored?

In archive :)
Example for Lenny:
deb http://archive.debian.org/debian/ lenny main non-free contrib
deb-src http://archive.debian.org/debian/ lenny main non-free contrib
# Security:
deb http://archive.debian.org/debian-security/ lenny/updates main contrib non-free
deb-src http://archive.debian.org/debian-security/ lenny/updates main contrib non-free
# Volatile:
deb http://archive.debian.org/debian-volatile lenny/volatile main contrib non-free
deb-src http://archive.debian.org/debian-volatile lenny/volatile main contrib non-free
# Backports:
deb http://archive.debian.org/debian-backports lenny-backports main contrib non-free